The present invention relates generally to the field of sensors. More particularly, the invention provides a method and system for automatic gain control of sensors. Merely by way of example, the system has been applied to the Electron Bombarded Charge Coupled Device (“EBCCD”) or other like sensor having a photo emitter that is biasable, gain control, which can be turned off and on, and way for reading out photocounts per pixel for a predetermined time. It would be recognized that the invention can be applied to other sensors that need automatic control, including sensors which can be biased to control the sensor gain.
Optical sensors have been widely used to detect signals at various wavelengths. The wavelengths may cover the ultraviolet band, the visible band, the infrared band, and others. In these bandwidths, the sensors should provide high-sensitivity and low-noise detection. To achieve the desired performance, extensive research has been conducted to design and improve optical sensors.
A type of optical sensor is the Electron Bombarded Charge Coupled Device (“EBCCD”). The EBCCD sensor can provide high-sensitivity and low-noise detection. Additionally, the EBCCD sensor can perform high-rate data transmission, and have a miniature size. Consequently, the EBCCD sensor is useful for advanced optical systems such as one in the Airborne Laser System of Lockheed Martin Corporation. As merely an example, the EBCCD sensor can be used for detecting airborne targets of a very low indication through a telescope of a moving aircraft or the like. Further details of the EBCCD sensor are provided below.
FIG. 1 is a simplified diagram for the conventional EBCCD sensor. EBCCD sensor 10 includes at least photocathode 20 and CCD focal plane array 30. CCD focal plane array 30 usually has a plurality of pixels in order for EBCCD sensor to detect incoming photons in various spatial locations.
FIG. 2 is a simplified diagram for a conventional focal plane array (“FPA”) of EBCCD sensor. As shown in FIG. 2, FPA 150 may include a large number of pixels. For example, FPA 150 has 16,384 pixels, each pixel occupying a 30-μm by 30-μm area. These pixels are grouped into 16 digitized read-out regions. Each read-out region contains 1,024 pixels and contains its analog to digital converter. Signal electrons in each read-out region can be swept out simultaneously and rapidly processed in parallel. For example, pixel charges in each read-out region are digitized and serially read-out as 1024 digitized “counts” within a short, typically 50-μs period. Simultaneously, pixel charges in other read-out regions are also read out and sent for computer analysis and storage.
As discussed above, the EBCCD sensor provides high-performance detection. Unfortunately, the EBCCD still has many limitations. For example, the EBCCD sensor is often difficult and expensive to make. As compared to other types of conventional sensors, which are relatively inexpensive, the EBCCD sensor can cost tens of thousands of U.S. dollars. Although the EBCCD sensor has high sensitivity and performance characteristics, such high sensitivity leads to damage from overexposure to light, which causes a shortened lifetime. The damage may occur at the focal plane array or other elements of the EBCCD. Still other problems include pixel saturation, pixel array blooming, and the like during operation. These and other limitations are described throughout the present specification and more particularly below.
Hence it is desirable to improve the EBCCD sensor.